Matrix isolation and solvation studies of reactive ... · Matrix Isolation and Solvation Studies of...

Matrix Isolation and Solvation Studies

of

Reactive Intermediates

Dissertation

Submitted to the Faculty of Chemistry and Biochemistry,

Ruhr-Universität Bochum to fulfil the requirements for the degree of

Doctor of Natural Science (Dr. rer. nat.)

Presented by

Soumya Radhakrishnan

From Kerala, India.

Bochum 2017

This work was carried out from October 2013 to July 2017 under the supervision of

Prof. Dr. Wolfram Sander at the department of Organic Chemistry II, Ruhr-Universität

Bochum, Germany.

Part of the work included in this thesis was also carried out at the Science Division of

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive,

Pasadena, USA under the supervision of Dr. Murthy S. Gudipati.

I would like to thank Prof. Dr. Wolfram Sander and Dr. Murthy S. Gudipati for their

constant supervision, encouragement and fruitful discussions on the projects

undertaken. I would also like to thank them for providing me with all the necessary

research facilities required for my scientific development.

First Referee: Prof. Dr. Wolfram Sander.

Second Referee: Dr. Murthy S. Gudipati.

Dissertation submitted: 12/10/2017

Disputation: 08/12/2017

Partial results of this work were already published in the following contributions:

Publications

“Photo-induced Reversible Electron Transfer Between the Benzhydryl Radical and Benzhydryl Cation in Amorphous Water-Ice.”

Soumya Radhakrishnan, Joel Mieres Perez, Murthy S. Gudipati, and Wolfram

Sander, J. Phys. Chem. A. 2017, 121, 6405.

“Photochemistry of Pyrene in H2O/CO2 ice.”

Soumya Radhakrishnan, Wolfram Sander and Murthy S. Gudipati

-in preparation.

“On the Matrix Isolation of Tropyl Radical and Cation.”

Soumya Radhakrishnan, Pritam Eknath Kadam and Wolfram Sander

-in preparation.

Scientific Meetings

Graduate School Solvation Science GSS autumn workshop, Ruhr-Universität

Bochum, 09-10.10.2014. (Poster)

Graduate School Solvation Science GSS workshop Area A, Ruhr-Universität

Bochum, 02.12.2014. (Poster).

Graduate School Solvation Science GSS Summer School. Ruhr-Universität

Bochum, 26-29.05.2015. (Poster)

Gordon Research Conference, Physical Organic Chemistry, Holderness School,

New Hampshire, USA, 21-26.06.2015. (Poster and Talk)

15th European Symposium on Organic Reactivity (ESOR 2015), Kiel, Germany,

30.08-04.09.2015. (Poster)

Graduate School Solvation Science GSS workshop Area A, Ruhr-Universität

Bochum, 26.10.2015. (Poster).

International Conference on Reactive Intermediates and Unusual Molecules

(ISRIUM 2017), Sorrento, Italy, 18.-22.6.2017. (Poster)

International internship

At the Science Division, Jet Propulsion Laboratory, California Institute of

Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA under the supervision

of Dr. Murthy S. Gudipati from 13.06.2016 – 14.09.2016.

"There will come a time when you believe everything is finished.

That will be the beginning"

Louis L’Amour.

Dedicated to

My Parents

Acknowledgements

First, I would like to convey my sincere gratitude to Prof. Dr. Wolfram Sander for

believing in me and giving me this opportunity to carry out my Ph.D. studies here in

Bochum, Germany. The constant appreciation and guidance throughout my scientific

journey in Bochum have helped me in developing not only my scientific knowledge but

also my outlook towards life. I am really indebted to him.

I would also like to express my sincere gratitude to Dr. Murthy S. Gudipati for

constantly guiding me not only scientifically but also personally like a true teacher. The

constant scientific discussions in the lab really increased my passion and thirst for

scientific research. The quality that I admire most in him is his passion and commitment

towards science and earnestness to let it reach people from all walks of life. His caring

nature both professionally and personally makes me admire him even more. With

regard to this opportunity, I would also like to express my sincere gratitude to Dr. Karin

Hassenrück for being a kind-hearted person and making me feel at home and

comfortable during my stay at La Canada Flintridge, USA. I found a friend in her with

whom I could sit and talk over any topic in a carefree manner.

I would also like to express my sincere gratitude to Frau Hannah Grasse, Frau Sabine

Weiß and Frau Jacinta Essling for helping me with all administrative work at RUB,

Germany. I would also like to thank Frau Arpine Margaryan for helping me with the

administrative procedures at JPL, USA.

I would like to thank all the present and past members of Prof. Sander’s group for

helping me in my scientific research and for creating a very conducive environment for

research. I would also like to sincerely thank Dr. Saonli Roy for not only helping and

guiding me with the pyrolysis experiments in the early stages of Ph.D. but also for

creating a homely environment at Bochum during the first few months of my stay. The

constant encouragement and motivation provided by her during my Ph.D. helped me to

sail through this journey with ease. I would also like to thank the members of Dr.

Murthy S. Gudipati’s group for helping me with the instrumentations in the lab and

making my stay and research in the lab comfortable during the three month visit.

I would like to thank Frau Heidemarie Spieß-Hack, Herr Klaus Gomann, Frau Catharina

Fulde, and Herr Robin Giebmanns for providing me with the necessary technical

support in the matrix and synthesis lab. I would also like to thank Herr Torsten

Haenschke for providing me with IT related support.

I would also like to thank Dr. G. Prabusankar, my M.Sc. thesis supervisor for his

constant support and words of wisdom from my M.Sc. days. I would also like to

sincerely thank Dr. Kavitha Velappan for introducing me to the OC II group and

encouraging me to do a Ph.D. under Prof. Dr. Sander’s supervision.

Last but not the least I would also like to mention all the people who helped me in proof

reading my thesis. They are Tanmoy Chakraborty, Priyadarshi Chowdhury, Jyoti

Campbell, Dr. Saonli Roy, Corina Pollock, Dr. Pritam Eknath Kadam, Smitha Preejith,

Iris Trosien and Lakshmy Balu.

Thanks to one and all for being there with me during my Ph.D. journey.

Table of Contents

Abstract 1

1. General Introduction 2

1.1. Radical ................................................................................................................. 2

1.2. Chemistry in Space .............................................................................................. 4

1.2.1. PAHs in space .......................................................................................................................... 5

1.3. Solar System Ices ................................................................................................. 7

1.4. Matrix Isolation .................................................................................................. 11

1.4.1. Technique and equipment ...................................................................................................... 13

1.4.2. Generation of the reactive intermediates................................................................................ 14

1.5. Quantum Chemistry ........................................................................................... 15

1.5.1. Density functional theory ....................................................................................................... 16

1.5.2. Basis sets ................................................................................................................................ 17

2. Benzhydryl Radical 18

2.1. Introduction ........................................................................................................ 18

2.2. Results and Discussions ..................................................................................... 20

2.2.1. Matrix Isolation and spectroscopic characterization of 1,1,2,2- tetraphenylethane ............... 20

2.2.2. Matrix isolation and spectroscopic characterization of the benzhydryl radical. .................... 23

2.2.3. EPR studies of the benzhydryl radical ................................................................................... 27

2.2.4. Photochemistry of the benzhydryl radical.............................................................................. 30

2.2.5. Photoionization in amorphous water ice ................................................................................ 31

2.3. Conclusion ......................................................................................................... 36

3. Benzhydryl Cation 37

3.1. Introduction ........................................................................................................ 37


3.2.1. Matrix isolation and spectroscopic characterization of the benzhydryl cation ...................... 38

3.2.2. Energetics of photoionization in amorphous water-ice .......................................................... 44

3.2.3. Photo-induced reversible electron transfer between benzhydryl radical and cation .............. 46

3.2.4. Reversible photo-induced electron-ion recombination in amorphous water-ice .................... 48

3.2.5. Quantification during the interconversion between the radical and the cation ...................... 50

3.2.6. Prolonged irradiation of the benzhydryl radical ..................................................................... 53

3.2.7. Electron Irradiation of the benzhydryl radical in amorphous water-ice ................................. 54

3.3. Conclusion ......................................................................................................... 57

4. Pyrene in CO2 Ice 58

4.1. Introduction ........................................................................................................ 58


4.2.1. Matrix isolation and spectroscopic characterization of pyrene .............................................. 59

4.2.2. Photochemistry of pyrene in H2O/CO2 ice ............................................................................. 62

4.2.3. Photochemistry of pyrene in pure CO2 ice ............................................................................. 66

4.2.4. Reaction scheme to support the photochemistry .................................................................... 72

4.2.5. Photochemistry of pure CO2 ice ............................................................................................. 74

4.3. Conclusion ......................................................................................................... 76

5. Tropyl Radical 78

5.1. Introduction ........................................................................................................ 78

5.2. Results and discussions ...................................................................................... 79

5.2.1. Matrix isolation and spectroscopic characterization of bitropyl ............................................ 79

5.2.2. Photochemistry of bitropyl .................................................................................................... 81

5.2.3. Matrix isolation and spectroscopic characterization of the tropyl radical. ............................. 84

5.2.4. EPR studies of the tropyl radical............................................................................................ 90

5.2.5. Reaction of the tropyl radical with molecular oxygen ........................................................... 92

5.3. Conclusion ......................................................................................................... 95

6. Tropyl Cation 96

6.1. Introduction ........................................................................................................ 96


6.2.1. Generation of the tropyl radical in amorphous water-ice matrix ........................................... 97

6.2.2. Photoionization of the tropyl radical in amorphous water-ice ............................................... 98

6.3. Conclusion ....................................................................................................... 102

7. Cyclopentadienyl Radical 104

7.1. Introduction ...................................................................................................... 104

7.2. Results and Discussions ................................................................................... 106

7.2.1. Matrix isolation and spectroscopic characterization of the cyclopentadienyl radical .......... 106

7.2.2. EPR studies of the cyclopentadienyl radical ........................................................................ 110

7.2.3. Reaction of the cyclopentadienyl radical with molecular oxygen ....................................... 112

7.2.4. Reaction of the FVT products of nickelocene with water .................................................... 118

7.3. Conclusion ....................................................................................................... 120

8. Summary 122

8.1. Benzhydryl Radical and Cation. ...................................................................... 122

8.2. Pyrene in CO2 Ice ............................................................................................. 123

8.3. Tropyl Radical and Cation ............................................................................... 124

8.4. Cyclopentadienyl Radical ................................................................................ 125

9. Material and Methods 127

9.1. Synthesis .......................................................................................................... 127

9.2. Analytical Equipments ..................................................................................... 127

9.3. Matrix Isolation ................................................................................................ 127

9.3.1. Apparatus ............................................................................................................................. 127

9.3.2. Deposition conditions .......................................................................................................... 128

9.3.3. Flash Vacuum Thermolysis (FVT) ...................................................................................... 128

9.3.4. IR and UV spectrometer ...................................................................................................... 129

9.3.5. EPR spectrometer ................................................................................................................ 129

9.3.6. Light Sources ....................................................................................................................... 129

9.3.7. Electron gun ......................................................................................................................... 130

10. Synthesis 131

10.1. Sodium salt of Diphenylcyclopropenone ....................................................... 131

10.1.1. 2,3-diphenylcyclopropenone tosylhydrazonium chloride .................................................. 131

10.1.2. 2,3-diphenylcyclopropenone tosylhydrazone .................................................................... 132

10.1.3. Sodium salt of 2,3-diphenycyclopropenone tosylhydrazone ............................................. 132

10.2. Lithium Salt of Tropone Tosylhydrazone ...................................................... 133

10.2.1. Synthesis of 1,1-dichlorocycloheptatriene ......................................................................... 133

10.2.2. 2,4,6-cycloheptatrienone p-toluenesulfonylhydrazone hydrochloride ............................... 134

10.2.3. 2,4,6-cycloheptatrienone p-toluenesulfonylhydrazone ...................................................... 134

10.2.4. Lithium salt of tropone tosylhydrazone ............................................................................. 135

10.3. Bitropyl .......................................................................................................... 135

11. Optimised Geometries 136

12. References 146

1

Abstract

Co-existence of organic matter and water is one of the prerequisites for life to

have evolved on earth. Organic chemistry in ice medium helps us understand the

processes of cryosolvation and radiation biology. Thus, photoionization of organic

molecules to generate highly reactive species is the center of many complex chemical

reaction pathways both on Earth and in a wide variety of environments in our Solar

System and beyond.

Organic radicals and polycyclic aromatic hydrocarbons (PAHs) are readily

ionized in low density amorphous (LDA) water-ice matrix, as the ionization potential

of the neutral compounds is greatly reduced. Consequently, the cations produced as a

result of this ionization are also greatly stabilized by up to 2 eV.

In this work, an intensive study of the benzhydryl radical, the tropyl radical and

pyrene in amorphous water-ice matrices was performed. The easy ionization of these

radicals and pyrene by irradiation with various light sources to form the corresponding

cations in amorphous water-ice matrix was studied. The benzhydryl radical and the

tropyl radical were generated through flash vacuum thermolysis (FVT) of 1,1,2,2-

tetraphenylethane and bitropyl, respectively, in argon, 1% CH2Cl2 doped argon and

amorphous water-ice matrices and were characterized using IR, UV-vis and EPR

spectroscopy. The formation of the benzhydryl cation and the tropyl cation from the

corresponding radicals in different matrices were investigated in detail. A complete

study of the energetics of the photoionization in amorphous water-ice and the reversible

electron-ion recombination in water-ice was undertaken for the benzhydryl radical. A

unique way to generate a cation using nonhalogenated precursors is also described.

Pyrene was co-deposited with varying mixtures of H2O and CO2. Consequent

vacuum ultraviolet (VUV) irradiation led to the formation of the pyrene cation along

with other photoproducts which were characterized using IR and UV-vis spectroscopy.

A comparative study of the nature of the photoproducts formed on irradiating ices with

different concentrations of H2O and CO2 was performed.

Similarly, the cyclopentadienyl radical was generated through FVT of

nickelocene and analyzed using IR, UV-vis and EPR spectroscopy. The reactions of the

cyclopentadienyl radical with small molecules like oxygen and water were performed

and the reaction pathways were studied in detail.

Thus, a complete study of the radicals and PAHs in argon and water matrices

showed the easy photoionization of these reactive intermediates in amorphous water-

ice matrices due to the lowering of their ionization potential by the water-ice.

Chapter 1 General Introduction

2

1. General Introduction

1.1. Radical

A radical is a chemical species with an unpaired electron in one of the orbitals

in the closed shell configuration. It is this single unpaired electron that determines the

fate of the radical in terms of the reactivity. While most of the radicals are reactive,

there are few radicals which are extremely stable for example, (2,2,6,6-

tetramethylpiperidin-1-yl)oxyl (TEMPO) and the phenylenyl radical, to name a few.

However, most radicals are stable under matrix isolation conditions of low temperature

and pressure. Radicals are usually formed via the homolytic bond cleavage of the

precursor species either thermally or photochemically. The presence of radicals and

their importance has been well documented in various fields of chemistry like

astrochemistry, plasma chemistry, biochemistry and in processes like combustion,

polymerization etc.

The term ‘radical’ was coined by Louis-Bernard Guyton de Morveau in 1785

following which Antoine Lavoisier in 1789 used this phrase for the first time in Traité

Élémentaire de Chimie.1 Gomberg was the first chemist to identify and report the first

free radical, the triphenylmethyl radical 1.1.2

Figure 1.1. The first reported stable radical triphenylmethyl radical 1.1.

Soon after the discovery of 1.1, various other radicals were discovered and

reported in solution phase.3-4 Since most radicals tend to recombine when in solution,

they could not be studied for spectroscopic reasons except under gas phase or matrix

isolation conditions. However, their use in different fields of chemistry is elaborated in

the following paragraphs. They are:

i) Combustion. This involves an elevated temperature exothermic reaction between the

reductant and oxidant. In most cases, the oxidant is molecular oxygen (O2). Thus,


3

combustion involves a series of complicated radical reactions. When O2 is used as the

oxidant, it undergoes bond cleavage to form oxygen radicals (O˙) in the singlet state.

The external heat supplied acts as an initiator for this bond dissociation. The oxygen

radicals now oxidize the reductants to form the oxidized products like the oxy or peroxy

radicals. PAHs can either undergo oxidation in the presence of small molecules like

H2O to form oxidized products like the hydroxy-PAHs or just burn and disintegrate into

smaller carbon components eventually forming soot.5-6

ii) Polymer Chemistry. Polymerization7 is a process wherein the reactive monomer

units combine with one another to form long continuous chains or 3-D networks

through a chemical reaction. The process of polymerization can occur through a step-

growth process or a chain-growth process.

Step growth polymerization process usually takes place when monomer units

contain functional groups like -OH, -CN etc. These functional groups initiate the

condensation reaction and long chains or 3-D networks of polymers is thus formed.

Most of the step growth polymers are condensation polymers.

Chain growth polymerization process takes place when unsaturated monomer

units are used. The unsaturated bond breaks and allows for the easy attack of another

monomer unit via the formation of a propagating bond. For example, the formation of

the polyethylene occurs via the radical polymerization through the π bond of ethene.

The π bond of ethene is broken and the two electrons formed then attack the unsaturated

bond of another ethene. This process occurs continuously to form the polymer. Thus,

most of the chain growth polymers are formed by radical polymerization through three

steps of chain initiation, chain propagation and chain termination.

iii) Biology. Free radicals are considered to be one of the most crucial reactive species

in many biological processes. Of the many free radicals present in biological processes,

the superoxide and the hydroxyl radicals are considered to be the most important. They

are produced from the reduction of molecular oxygen. The concentration of these

radicals determines the fate of the cell. An excess of these free radicals pose a serious

threat to the longevity of the cell as they lead to unwanted side reactions, ultimately

causing cell damage. Various diseases like the Parkinson’s disease, myocardial

infraction, diabetes, cancer and stroke occur due to an excessive amount of the

unwanted free radicals.


4

1.2. Chemistry in Space

The prerequisites for life to evolve on earth is the co-existence of organic matter

and water. Though we still do not understand how exactly life originated, other

necessary conditions are understood to be energy and minerals. Interestingly, all of

these four conditions are already met even before our Solar System even formed–during

the interstellar molecular cloud phase. Interstellar ice grains of micron-size are

constantly bombarded with cosmic rays and photons from nearby stars. These

interstellar ice grains8 are composed of a micron-sized silicate (mineral) dust nucleus

upon which water, carbon monoxide, carbon dioxide, ammonia, methanol, sulfur-

containing molecules such as OCS, and perhaps even large polycyclic aromatic

hydrocarbons (PAHs) condense forming the interstellar ice grain.9 Photochemical

evolution of the interstellar ice grains is an important branch of astrochemistry and

tremendous progress has been made in the past few decades that shows that building

blocks of life could have already been produced in the interstellar ice grains.9-10 The

dense molecular clouds in the interstellar medium collapse forming the first phase of a

star and a solar system–called protostar and protoplanetary disk.11 These ice grains then

transform into planets, moons, asteroids, and the reservoir comets at the outer rim of

the Solar System, known as Kuiper Belt Objects (KBOs).12-13 Complex organic matter

made in the interstellar medium and preserved through KBOs, comets and asteroids

could have been delivered to Earth14-17 during the early stages of our Solar System

formation (about 4 billion years ago),18 which coincides with the time during which life

first evolved on earth as determined through dating bacterial fossils.19

Ionized molecules are energy-rich and highly reactive species, making further

chemical reactions mostly barrier-less processes. Understanding radiation-induced

chemical pathways of organic matter in a water-dominated ice medium is also important

for our understanding of how pollution affects the Earth’s cryosphere (polar ice caps to

snow and glaciers to cirrus clouds dominated by ice grains).20-22 The photoionization of

organic molecules to generate highly reactive species is the center of many complex

chemical reaction pathways both on the Earth and in a wide variety of environments in

our Solar System and beyond.23-24 Gaseous and particulate matter in the form of

interstellar clouds dominate the interstellar space. These clouds usually have a

temperature of 50 K-100 K and a density of around 10-1000 cm-3. Their dense cold

regions are dominated by organic matter mainly of molecular origin such as around 120

species have been identified surrounding old low-mass stars.25 The organic matter

found in the interstellar clouds is further divided into two categories. One in which the


5

molecules found are extremely common like H2O, NH3 etc. and the other category

consists of charged species like the H3+, HCO+ etc., or the radicals (CnH or more

complex organic species like the PAHs and isomers). H2 is the most abundant species

followed by CO. These interstellar clouds later collapse to form stars and planetary

systems. Out of the complex organic species PAHs, cations and radicals are among the

few molecules which have gained attention in the recent past.

1.2.1. PAHs in space

Polycyclic aromatic hydrocarbons (PAHs) are a class of hydrocarbons

containing extensively delocalized fused aromatic rings of carbon and hydrogen only.

These fused aromatic rings do not have any substituents attached to it. Thus, they

mainly consist of two or more fused benzene rings which are bonded in linear, cluster

or angular arrangements as shown in Figure 1.2.

Figure 1.2. Different types of polycyclic aromatic hydrocarbons (PAHs) based on their

molecular arrangement of the fused benzene rings.

PAHs are not only classified based on their molecular arrangement but also on

the number of benzene rings present in the structure. These are the small PAHs and the

large PAHs. The small PAHs typically consist of up to six fused aromatic rings in any

molecular arrangement whereas the large PAHs consist of more than six fused aromatic

rings. Apart from having a high melting and boiling point, the characteristic properties

such as the volatility and solubility of the PAHs are determined by their molecular

weight. Thus, with every additional benzene ring to the parent structure the vapor

pressure decreases and the solubility in aqueous solution also decreases. These PAHs

also have a very characteristic UV absorption spectrum which makes their identification

easy.

PAHs are formed by the partial combustion of organic matter, for example,

during the combustion of fossil fuels and biomass burning5-6 as a part of human


6

activities. They are also very much present in the interstellar medium. Studies on the

detection and the interaction of PAHs with small abundant molecules like CO, H2O etc.

in interstellar medium have been completed.26-27 The presence of PAHs especially

pyrene C16H10 in the comets, the last forming body in the solar system further shows

the importance of PAHs in space.28-31 PAHs necessarily freeze out onto the dusty ice

grains during the molecular cloud formation. Apart from PAHs, these dust grains also

contain molecules such as H2O, CO, CO2, NH3, CH3OH to name a few.32-33 Subjected

to intense UV radiation from the field formed by the interaction of the cosmic rays with

hydrogen atoms present in the gaseous medium, PAHs readily ionize to form complex

species of scientific interest.34-35 Gudipati et al. were the first to report the VUV induced

photoionization of the PAHs to form PAH-radical-cations in water-ice mimicking the

action of VUV on ice grain mantles containing PAHs.36-37 Following this lead, several

laboratory studies on the ionization of PAHs in different ice analogues were performed

to gain insight into the different photochemical process taking place in the ices of the

interstellar medium (ISM).38-41

Theoretical models of the interstellar grain mantles formed by the accumulation

of dusty ice grains help us to understand its composition.42-43 Three theoretical models

were prepared. Model 1: If the abundance of hydrogen atoms in the gas phase are

greater than other molecules in the ISM like CO, O and O2, hydrogenation reactions

take place. Hence, a polar ice mantle will be formed with hydrogenated products like

H2O and H2CO. Formation of a non-hydrogenated species like CO2 will be sparse.

Model 2: if the abundance of hydrogen atoms is less in comparison to the heavier

species like CO, O and O2 with O2 being the most abundant species then the formation

of oxygenated species especially CO2 will be high. Model 3: High abundance of oxygen

in atomic form would lead to the abundance of a less polar oxygenated species like CO,

CO2, O3 etc.

Looking at the theoretical models mentioned above, one cannot overrule the

abundance of CO2 in ice grain mantles together with H2O, CO, O3 etc. These icy grain

mantles are the basic units for the formation of stars and solar systems. The extreme

low temperatures (10–20 K) of the grain mantles in the ISM force the atoms/molecules

in it to clump together to form high density masses which then collapse due to their

own weight to form the protostar (the first stage of the star formation). This protostar

evolves in due course of time to form young stars and solar systems. Further,

astronomical explosions (supernova) facilitate the formation of more stars with changed

composition (ionization) due to shock waves. Thus, the stars formed with an abundance


7

of CO2 and various other gases are subjected to various photo physical process and lead

to the formation of new photoproducts. These stars eventually form planets, moons,

asteroids etc., depending on their density.

As discussed, PAHs are present in the interstellar medium26-27, 44-45 and in turn

in comets,28 the surfaces of Saturn’s moons, Hyperion and Iapetus46-47, also as complex

organic substances in Saturn’s rings,48 in meteoritic samples49 and in interplanetary dust

particles50. The unknown fluorescence band detected in the range of 280-400 nm in the

coma of Halley’s Comet and the extended red emission has confirmed the presence of

PAHs in outer space. The UV spectral identification of the red rectangle nebula with

that of anthracene and pyrene,51-53 and also the presence of pyrene in Halley’s comet28

further strengthens the evidence of PAHs in space.

1.3. Solar System Ices

H2O exists in three different states of matter in our planet, Earth. They are solid,

liquid and gas. Generally, the term “ice” refers to H2O in the solid state on Earth.

However, any volatile gas frozen in our solar system is termed as “ice” when one

considers the planetary science studies. Our solar system is made up of primary

elements like hydrogen, helium, carbon, oxygen and nitrogen. Molecules like H2O, H2S

and CH4 are found in abundance due to the reducing nature of the atmosphere of the

solar system. Apart from the formation of H2O and other molecules by the reducing

atmosphere in the solar system, the presence of CO, CO2 and other molecules formed

by some oxidizing reactions cannot be ruled out. These molecules condense in the

atmosphere and participate in the planet formation due to their presence in the

protoplanetary disk or in the nebulae. Thus, in planetary science studies, diverse types

of ices are studied like H2O ice, SO2 ice, N2 ice and CH4 ice to name a few.

i) H2O ice: As already known H2O has three fundamental infrared active vibrations in

the gas phase at 3755.1, 3652.3 and 1594.8 cm-1. They correspond to the asymmetric

O-H stretch, symmetric O-H stretch and H-O-H bend respectively. However, these

values are shifted in liquid water. In planetary science studies, H2O in the form of ices

is of utmost interest and importance for the study on the evolution of ices in the planet

systems. Thus, out of the distinct phases of water the amorphous and the crystalline ices

are given priority. The comets, icy bodied planets and satellites are indigenously

composed of these ices. The detection of H2O ice using IR spectroscopy technique is


8

extremely easy due to the strong and broad absorptions of H2O around 3500 cm-1 to

3000 cm-1 (Figure 1.3).

Figure 1.3. IR spectrum of amorphous water-ice isolated at 3 K.

The H2O ices generated on the cold spectroscopic window at a low temperature

(3 K) and low pressure (ultrahigh vacuum) is of utmost interest in the study of ices in

the solar system as their presence has been confirmed in the interstellar medium thereby

helping in the formation of protostars and eventually stars. The amorphous and the

crystalline ices are of immense importance as these phases have been identified during

the remote sensing experiments of the icy objects.

Amorphous H2O Ice: These ices are characterized by the presence of a broad

structureless absorptions around 3500 cm-1 to 3000 cm-1. The broadness of the O-H

band is probably due to the change in the frequency of the O-H bond caused by the

distortion in the hydrogen bonds among the H2O molecules. These ices are being

classified on various parameters and different names have been assigned based on these

classifications like the vitreous ice, amorphous solid water-ice, hyper-quenched glassy

water-ice, high density and low density amorphous ice to name a few.54 Out of these,

the ices differentiated based on the density will be discussed. The high-density

amorphous ices exist below 70 K whereas the low density amorphous ices exist between


9

70 K to 120 K. Usually amorphous ices are formed when water in the form of water

vapor are deposited on to the cold spectroscopic window maintained below 120 K.

Amorphous ices deposited at temperatures below 70 K have weaker bands at higher

wavenumbers in comparison to the ones deposited at temperatures above 70 K.

However, annealing and cooling back these ices result in the loss of their original

characteristic feature thereby accounting for an irreversible change. For our

convenience in the thesis, we use the term “amorphous ice” to represent both the low

density and high density amorphous water-ices.

Crystalline H2O Ice: Usually crystalline ices are formed when water vapors are

deposited on a cold spectroscopic window maintained at temperatures above 120 K or

when the amorphous ices are annealed to temperatures above 120 K. In sharp contrast

to the amorphous ices, these ices are characterized by sharp intense bands in the OH

stretch region of the water.

H2O ice has been detected in the interstellar medium.55 Out of the many spatial

bodies detected with H2O ice, few are mentioned here. Crystalline ice has been detected

on Charon (Pluto’s satellite),56-57 Triton (Neptune’s satellite).58-59 Hansen et al.60 found

the presence of both crystalline and amorphous H2O ice on Jupiter’s largest moon,

Ganymede whereas only crystalline ice was detected on Callisto, Jupiter’s second

largest moon. Europa also contains H2O in form of ices, gullies and mantles. Rivkin et

al.61 also reported the presence of H2O in the asteroid 24 Thermis. Comets are also said

to contain H2O ice but the absorptions were found to be extremely weak.62 Thus, the

abundance of both amorphous and crystalline water-ices in our Solar system paves the

way for its extensive study in the laboratory.

Figure 1.4. The Mars Express took this photo of a crater on Mars filled with water-ice.

Credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO

ii) SO2 ice: Found in Venus’s atmosphere, this is also differentiated as amorphous and

crystalline ices.63 They are present in the Venusian atmosphere as clouds and haze of


10

sulphuric acid. Sulphuric acid is formed by the reaction of SO2 ice with the H2O present

in the atmosphere.

iii) N2 ice: Experimentally, it has been found out that N2 sublimes at 40 K from the

surface of H2O ice. Thus, for N2 ice to form in the solar system, extreme low

temperatures are required. Hence, N2 ices can be found in the outer regions of the solar

system especially beyond the Oort cloud where are temperatures are extremely low. N2

ice exists as α-N2 ice and β-N2 ice. At T > 35.61 K, N2 ice exists as β-N2 ice.64 The

absorption coefficients are too weak in comparison with the other ices to observe

notable absorptions. However, the peaks due to β-N2 ice become narrower on

decreasing the temperature. α-N2 ices, on the other hand, are found at temperatures

lower than 35.61 K. A change in the shape of the bands is noticed although the

absorption coefficients of α-N2 ice are the same as the β-N2 ice.65 They are found

seasonally in the outer edges of the solar system where the temperature remains below

35.61 K. N2 is difficult to detect via infrared spectroscopy because of its non-polarity.

The fact that α-N2 ice and β-N2 ice are observable is due to the collision with other N2

molecules creating an induced dipole change. This favors the detection of N2 ices

although their absorption coefficients are weak. N2 ice are predominantly found on the

surface of Pluto. Traces of CH4 ices are present on the surface either diluted in the N2

ice matrix or present separately. The New Horizons Mission by NASA was the first

ever mission launched to study the surface of Pluto in detail and other objects present

in the Kuiper belt. The presence of a nitrogen atmosphere, distinct surface engravings,

ice rock interior and the discovery of smaller moons in the Kuiper belt was the main

outcome of this mission.

iv) CO2 ices: Along with the presence of H2O ice, Mars is also dominated by CO2 ices

mainly in the poles as polar ice caps. During the pole’s winter, the pole is away from

the sun’s radiation and the temperature drops well below normal. Because of this all the

CO2 present previously in the atmosphere in the gaseous form condenses and forms a

layer of thick CO2 ice (often known as dry ice). In summer, the CO2 ice on these poles

evaporates due to the elevated temperature and it produces strong winds of CO2, H2O

vapor and dust. However, the percentage of CO2 present as ice is very small in

comparison to the water.

Solid CO2 is commonly found in young stars.66-67 However, the nature of the

CO2 ice–whether it is crystalline or amorphous–is not known.


11

Amorphous CO2 ice: As reported in the literature, so far amorphous CO2 ice has

not been detected in space. CO2 grown at extreme low temperatures (T = 8 K) and in an

ultrahigh vacuum chamber leads to the formation of amorphous CO2 ices. This is

denoted by the presence of the "low frequency shoulder" band at 2343 cm-1.68 These

bands are observed in the thin layers of CO2 ices. Absence of this band in the IR spectra

of dense clouds obtained from various sources show that the CO2 present in space does

not consist of amorphous ice.69 Presence of a band at 655 cm-1 is also observed in

amorphous ices. These ices remain in the amorphous phase up to 30 K. The amorphous

ice is more porous, with densities that can change within a range of values.70

Crystalline CO2 ice: Annealing the amorphous ices above 30 K leads to the

change in the phase of CO2. CO2 transforms to a much more crystalline phase. This is

denoted by the disappearance of the "low frequency shoulder" band at 2343 cm-1, the

appearance of a strong band at 2343 cm-1 and the formation of the doublets at 660 and

655 cm-1 as opposed to the single peak at 655 cm-1 in amorphous ice. Low temperature

crystalline CO2 has a density of 1.78 g/cm3.

CO2 sublimes off at 85 K and the crystalline ices are seen until 80-85 K.

Thus, different type of ices mentioned above together with hydrocarbon ices are

present in the solar system stating that the term “ice” is rightly used for any volatile

substance that has been frozen.

1.4. Matrix Isolation

George Pimentel, the “father of matrix isolation” developed this field71-72 and

made extensive studies together with George Porter.73 Thus, for Pimentel, matrix

isolation meant the trapping of a reactive intermediate with a fleeting existence in a

large excess of an unreactive gas and condensing it onto a cold spectroscopic window

thereby immobilizing the reactive intermediate. This cold material is now believed to

be matrix isolated as the reactive intermediate are well isolated from one another

through layers of inert host gas.

However, in due course of time, various changes to the host materials have been

made. For example, the use of frozen solutions, polymers, glasses, zeolites or crystals.

The rigidity of the matrix formed further prevents the immobilization and the diffusion

of the reactive intermediates paving the way for complete isolation.


12

Figure 1.5. Matrix isolation of the rigid guest species (black triangle) in solidified host

matrix (white circle) on a cold spectroscopic window at 3 K.

Studies of the reactive intermediates can be done in two ways. They are i)

detection immediately after their formation or ii) detection of the trapped molecules

after a period of time with ease. Both of these methods have their own advantages.

While the former allows the kinetic study of the reactive intermediates, the latter allows

the electronic and the molecular structural identification studies of the reactive

intermediates.

The advantages of the use of matrix isolation techniques over other techniques

are i) use of the inert gas: The unreactive nature of the inert gas allows for temperature

dependent studies of the reactive intermediates due to the non-interference of the host

gas. ii) The transparency of the homonuclear diatomic gases in the IR and the UV-vis

range for the structural studies. These inert gases also provide high resolution to the

spectra allowing for decoding the complex structure much easier. iii) The generation of

the reactive intermediates either externally or internally. Externally, the reactive

intermediates can be generated via thermolysis or photolysis of the substrate and then

it can be deposited on to the cold spectroscopic window maintained at extremely low

temperatures of 3 K-10 K. These temperatures suppress any type of reaction within the

matrix except tunneling. Interestingly, there are certain limitations to the matrix

isolation method. They are i) the size and nature of the precursor. The precursor of the

reactive intermediate should be either highly volatile or volatile enough to prevent

decomposition. Thus, usually the size of the organic species should be small enough to

facilitate the vaporization of the precursor and deposition on to the cold spectroscopic

window. ii) Cage effect: The precursors used for the generation of reactive

intermediates via thermolysis might be trapped in the same matrix cavity which might

allow for their recombination. Thus, when spectroscopic studies are done, these species


13

might already be in the neutral form due to recombination. iii) Polar substrates: The

polar substrates have a high tendency towards aggregation in matrix gases due to which

isolation of the reactive intermediate is almost impossible.

1.4.1. Technique and equipment

The deposition of the reactive intermediates along with the host gases are done

on spectroscopic windows maintained at cryogenic temperatures of 3 K-10 K. Cesium

iodide CsI windows are used for the IR measurements whereas for the UV-vis

measurements, quartz or sapphire windows are used. For the EPR measurements, the

matrix is formed on an oxygen-free copper (Cu) rod maintained at 5 K. A highly diluted

sample mixture is taken for the matrix preparation. These matrices are frozen to restrict

the rotation of the bonds facilitating the IR and UV-vis measurements. The samples

used for the generation of the reactive intermediates are degassed by freeze-pump-thaw

method to remove any dissolved air in them. For an ideal deposition of the matrix on to

the window, the window is set at a temperature about 30% of the melting point of the

inert gas (argon, neon etc.) used. This is then cooled down to the lowest possible

temperature of the cryostat (3 K) for spectroscopic studies.

To build a matrix setup, many devices are required as shown in Figure 1.6. These

are mentioned below.

Figure 1.6. Schematic diagram of a closed cycle cryostat with all the equipment attached.


14

A closed cycle cryostat to maintain the low temperature of the spectroscopic

window. Here, Sumitomo Heavy industries two-staged closed-cycle helium cryostats

(cooling power 1 W at 4 K) is used to obtain temperatures around 3 K.

Sample holders and the spectroscopic window for the deposition and analysis

of the matrix using IR and UV-vis spectroscopy.

Inlet systems and sample preparation lines for the passage of the sample and the

host gas before depositing on the cold window.

Vacuum chamber for the entire matrix isolation setup.

A thermal shield to protect the window and the wirings from the heat of the hot

pyrolysis oven.

A gas line connected to the vacuum chamber with a flow controller to control

the flow of the host gas onto the spectroscopic window.

Spectrometers for the detection of the reactive intermediates.

1.4.2. Generation of the reactive intermediates

The reactive intermediates to be studied under matrix isolation conditions were

prepared by the following methods.

Externally either by vacuum ultraviolet (VUV) irradiation using a microwave

gas discharge lamp or by flash vacuum thermolysis (FVT) of the precursor. For the

VUV irradiation of the precursor externally, the precursor mixed with the host gases are

irradiated before depositing on the cold window for spectroscopic studies. Thus, the

precursor forms the reactive species which then deposits on the cold spectroscopic

window for analysis. For the discharge lamp, hydrogen is used as a carrier gas as it

emits not only Lyα (121.6 nm, 10.2 eV) but also includes emission due to a broad

molecular hydrogen peak near 160 nm. FVT is another method to generate reactive

intermediates from the precursor. The precursor is allowed to pass through a

thermolysis oven heated electrically with a tantalum wire. The thermolysis oven is set

at a particular temperature for the easy generation of the reactive intermediates from

the precursor which is then mixed with the host gas and deposited on the cold window

(Scheme 1.1). Here the reactive intermediate should be stable throughout the entire

length of the thermolysis oven. However, chances for recombination and further

fragmentation cannot be ruled out.


15

Scheme 1.1. The generation of the benzyl radical 1.6 from the precursor bibenzyl 1.5 in

solid argon via flash vacuum thermolysis (FVT).

In-situ generation by depositing the precursor with the host gas and irradiating

the matrix at 3 K with light of appropriate wavelength to generate the reactive species.

For example, the generation of carbenes photolytically from the diazirine precursor

(Scheme 1.2). For the generation of the reactive intermediates mercury arc Hg lamps,

microwave gas discharge lamp, lasers, light emitting diode LEDs and xenon Xe lamps

are used as the photolysis source.

Scheme 1.2. Generation of carbene 1.8 from the diazirine precursor 1.7 in solid argon via

Ar-discharge lamp.

Co-condensation of reagents. In this method, the metal atoms (evaporated from

Knudsen cell) are co-deposited with a host gas that has been premixed with a molecule

whose adduct with the metal atom should be studied. For example, the formation of

radical anions on the photoionization of metal containing host-molecule matrix. The

metal ions are easily excited and the electrons thus generated is readily taken by the

molecule to produce radical anions.

1.5. Quantum Chemistry

Theoretical quantum chemistry has been of immense help to spectroscopists as

it is with the help of this field that questions regarding the stability, reaction pathways,

mechanisms, ground states, excited states and transition states of individual atoms or

molecules can be studied. Together with other studies, quantum chemists study the

reaction pathways of the reactants and the products in the chemical reaction. For these

studies, methods based on the quantum mechanical principles are used. In most of the

methods, the Born-Oppenheimer approximation is used. Born-Oppenheimer


16

approximation states that the separation of the motion of nuclei and electrons is

possible. In this approximation, the heavy mass of the nucleus is considered and is

compared with the mass of the electron. Since, there is an attractive force between the

nucleus and the electrons, an external force on the atom causes a faster response/motion

of the electrons when compared to the nucleus. While the acceleration produced by the

external force is much less on the nucleus due to its high mass (acceleration is inversely

proportional to mass) when compared to the motion of the electron. The nucleus thus

seems to be stationary. Thus, the wavefunction only accounts for the position of the

nucleus and neglects the motion of the nucleus.

Although the discovery of Schrödinger’s equation is considered to be the

beginning of Quantum Chemistry. Heitler et al. published their article74 on the studies

of hydrogen molecule considering the chemical bond between the two hydrogen atoms.

This work was considered to be the first step towards the development of quantum

chemistry. Following which quantum chemistry was applied in solving various

unanswered questions about black body radiation (1859), cathode rays (1838) etc.

Solving the Schrödinger’s equation remains the basis for understanding any

quantum chemical problem. By doing so, the chemical property of the molecule of

interest can be well understood. The exact solution to the Schrödinger’s equation can

be obtained by using H2 as the molecule of interest. Since most of the species contain

more than one atom and hence more electrons, different approaches have been adopted

to solve the Schrödinger’s equation like the wave model method, valence bond method,

molecular orbital method and density functional theory.

1.5.1. Density functional theory

As described above, density functional theory (DFT) is a method which can

calculate the ground state of large molecules (many electron systems) by considering

their electron densities. Because of this, it is one of the most widely used methods in

computational chemistry.75 DFT was used for the first time by Walter Kohn and Pierre

Hohenberg in their Hohenberg-Kohn (H-K) theorems.

The first H-K theorem considers the electron distribution in all three co-

ordinates while calculating the ground state of a many electron system. This theorem

also extends itself for the development of time dependent density functional theory

(TDDFT). TDDFT is used for calculating, for example, the wavelength required for the

excitation of a species (excitation energy) and the photoabsorption spectra.


17

The second H-K theorem mentions the use of an energy functional for a system

which is responsible for the energy minimization of the correct ground state electron

density.

These above theorems formed the basis of DFT.

1.5.2. Basis sets

Basis sets are used during the calculations performed using DFT or Hartree-

Fock method as they represent a set of functions describing the electron wavefunction.

Geometry of the species of interest are optimized using different types of DFT

functionals (BLYP, B3LYP, M062X etc.) together with the calculation of second

derivatives for the characterization of several stationary points along the potential

energy surface. The smallest basis sets are called the minimal basis sets and represent

the situation of a gas phase atom. Polarization functions and diffuse functions are the

other additive functions used together with the minimal basis set for the inclusion of

polarization of the electron density of the atoms and the diffusion of the electron density

to the extreme end of the molecule far away from the nucleus respectively. Examples

of some basis sets with polarization functions and diffuse functionals are 6-311++G**

where ++ denotes the diffuse functions on heavy atoms and hydrogen while **

represents the polarization functions on heavy atoms and hydrogen. Other examples are

6-31+G*, 3-21+G*, 3-21++G etc.

Chapter 2 Benzhydryl Radical

18

2. Benzhydryl Radical

2.1. Introduction

Organic radicals and carbocations are fundamental reactive intermediates that

are of paramount interest to organic chemistry. In principal, these intermediates can

interconvert via single electron transfer (SET). Thus, the direct photoionization of free

organic radicals using short wavelength UV light or X-ray irradiation is a convenient

way to produce the corresponding carbocations. If the intensity of light is high enough

and the excited state lifetime long enough, two photon ionization of radicals with near

UV or visible light can become efficient. In a laser flash photolysis study, Faria and

Steenken demonstrated that 308 nm irradiation of the benzhydryl radical in acetonitrile

or aqueous ethanol produces the benzhydryl cation via two photon ionization.76 The

lifetime of the excited state of the benzhydryl radical in acetonitrile is 100–330 ns, long

enough to absorb a second photon that leads to ionization. The ionization yield increases

in the presence of n-butylchloride, which is able to trap the ejected electron and thus

prevents the recombination of the initially formed cation–solvated electron pair. To

produce carbocations 2 via single photon ionization the photon energy has to be larger

than the ionization potential of the radical 1, which requires vacuum UV or X-ray

irradiation.

Scheme 2.1. The generation of the benzhydryl cation 2a on irradiation of the radical 1a

with 308 nm light in acetonitrile.

Photoionization can also be used to generate carbocations under the conditions

of matrix isolation. Thus, vacuum UV irradiation, using an Ar-discharge light source,

of the phenyl radical 1b during deposition of an argon matrix produces the phenyl cation

2b, one of the most reactive carbocations (Scheme 2.2).77-78 In solid argon at cryogenic

temperatures benzhydryl cation is stable and could be characterized by IR spectroscopy.

In a very detailed study, Bally et al. used X-ray irradiation to ionize the allyl

radical 1c and the benzyl radical 1d to obtain the corresponding allyl and benzyl cation

2c and 2d, respectively.79 In these experiments, iodides R-I were used as precursors for


19

the radicals. The iodine atoms formed concomitantly serve as excellent scavenger for

the electrons produced during the ionization.

Scheme 2.2. The generation of carbocations from their corresponding radicals using VUV

irradiation under matrix isolation conditions.

Since the photoionization in the solid state produces charged species, the

polarity of the matrix host should have a considerable influence on the yield and

stability of the cation. Both recombination with the free electron and reactions with

reactive matrices reduce the yield of the cation. Thus, the phenyl cation 2b even reacts

with molecular nitrogen in a barrierless, highly exothermic reaction. Polar matrices

such as amorphous water-ice or organic glasses will stabilize the cation, but might also

lead to unwanted side reactions.

Here, the isolation and spectroscopic characterization of the benzhydryl radical

1a in both pure argon and amorphous water-ice matrices at 3 K via matrix isolation

techniques were studied. The photochemistry of the radical was also studied in detail.

Photoionization of the radical was done in apolar solid argon. For these studies, a highly

efficient radical source that avoids the generation of electron scavengers such as

halogen atoms as byproducts was developed. Thus, photolysis or thermolysis of halides

R-X, which are frequently used radical sources, was not advised. Instead, the flash

vacuum thermolysis (FVT) of 1,1,2,2-tetraphenylethene 4 with subsequent trapping of

the products in argon at cryogenic temperatures (4–20 K) to produce the radical 1a in

very high yield was used.


20

Figure 2.1. The structure of 1,1,2,2-tetraphenylethane 4.

2.2. Results and Discussions

2.2.1. Matrix Isolation and spectroscopic characterization of 1,1,2,2-

tetraphenylethane

The symmetry present in 1,1,2,2-tetraphenylethane 4 allows for the easy carbon-

carbon single bond cleavage leading to the formation of a pair of identical benzhydryl

radicals also known as benzhydryl radicals. Studies of structures with similar bond

breaking mechanisms for the generation of radicals have been reported.80

The sublimation temperature of 4 is around 130–135 °C at a pressure of

~ 10-5 mbar. 4, along with an excess of gas (which forms the matrix) was deposited onto

the cold spectroscopic window maintained at different temperatures according to the

matrix gas used in the experiment e.g., argon (deposited at 25 K) and amorphous water-

ice (deposited at 55 K). The window was then cooled down to 3 K for spectroscopic

studies. IR and UV-vis studies were performed.

The IR spectrum of 4 in pure solid argon shows very strong and characteristic

peaks at 610.6, 699.2 and 743.6 cm-1 and is in good agreement with the calculated

spectrum obtained at the B3LYP-D3/6-311++G(d,p) level of theory as well as with the

spectrum obtained from NIST, SDBS databases (Figure 2.2, Table 2.1). UV-vis

spectrum of 4 shows absorption bands at 219, 223, 248.6, 256.0, 262.0 and 269.1 nm

in argon matrix (Figure 2.3) while at 228, 249.2, 256.8, 261.5 and 269.2 nm in

amorphous water-ice matrix (Figure 2.4). The complex structure of 4 makes it difficult

to resolve the peaks in the UV-vis spectrum.


21

Figure 2.2. IR spectra of matrix isolated 1,1,2,2-tetraphenylethane 4 in argon matrix at

3 K in (500-1700) cm-1 range (above) and in (2400-3500) cm-1 range (below) a) IR spectrum

of 4 in argon matrix deposited at 25 K and recorded at 3 K. b) IR spectrum of 4 calculated

at the B3LYP-D3/6-311++G(d,p) level of theory.


22

Figure 2.3. UV-vis spectrum of 1,1,2,2-tetraphenylethane 4 in argon matrix deposited at

25 K and recorded at 10 K.

Figure 2.4. UV-vis spectrum of 1,1,2,2-tetraphenylethane 4 in amorphous water-ice matrix

deposited at 55 K and recorded at 10 K.


23

Table 2.1. Experimental and calculated vibrational frequencies of 1,1,2,2-

tetraphenylethane 4.

Mode Sym Calculateda

ν/cm-1 (Iabs)c

Argonb

ν/cm-1 (Irel)d

Assignment

27 A 574.6 (8.0) 566.0 (4.4) Skeletal vibrations.

29 A 618.5 (43.0) 610.6 (17.1) Skeletal vibrations.

37 A 716.0 (103.0) 699.2 (100) Ring C-H bend (out of plane)

42 A 770.3 (6.0) 743.6 (48.6) Ring C-H bend (out of plane)



56 A 981.6 (2.0) 924.6 (1.1) Ring C-H bend (out of plane) + C-C

stretch

70 A 1054.1 (3.0) 1033.2 (6.5) Ring stretch

75 A 1114.7(10.0) 1074.8 (6.2) Ring C-H bend (in the plane)

82 A 1191.0 (1.0) 1156.9 (0.7) Ring C-H bend (in the plane)



100 A 1407.6 (4.0) 1361.5 (0.3) C-H bend

101 A 1478.9 (9.0) 1452.4 (13.8) Ring C-H +ring bend

104 A 1486.7 (4.0) 1456.7

105 A 1525.5 (3.0) 1494.5

1497.9 (27.9) Symm. C-C stretch

114 A 1639.8 (13.0) 1603.1 (27.4) Symm. C-C stretch

118 A 3042.1 (12.0) 2904.4 (20.1) Symm. C-H stretch

119 A 3144.7 (8.0) 3009.6 (2.3) Ring asymm. C-H stretch




138 A 3213.8 (2.0) 3113.3 (14.3) Ring symm. C-H stretch a Calculated at the B3LYP-D3/6-311++G(d,p) level of theory. b In argon matrix at 3 K. c

Absolute intensities in km/mol. d Relative intensities based on the strongest observed

absorption band.

2.2.2. Matrix isolation and spectroscopic characterization of the benzhydryl

radical.

The benzhydryl radical 1a was generated in good yields by the FVT 4 at

(550-560) °C using the sublimation cum thermolysis oven in different matrices.

(i) In Argon matrix: The products of the FVT of 4 together with a large excess

of argon were deposited on to the cold spectroscopic window at 3 K (Scheme 2.3). IR,

UV-vis and EPR measurements were performed. The FVT of 4 is highly efficient, and

yields of 1a of >95% can be achieved. The only major contamination observed in the

spectra of these matrices is remaining precursor 4 (< 5%).


24

Scheme 2.3. Generation of the benzhydryl radical 1a by the FVT of 4 at (550-560) °C in

solid argon at 3 K.

The IR spectrum of matrix isolated 1a in argon shows the presence of strong

peaks at 1446.6, 777.8 and 675.9 cm-1 and is in good agreement with the calculated

spectrum obtained at the B3LYP-D3/6-311++G(d,p) level of theory. (Figure 2.5, Table

2.2).

Figure 2.5. IR spectra of the benzhydryl radical 1a in argon matrix at 3 K. a) IR spectrum

showing the formation of 1a in argon at 3 K by FVT of 4 at (550-560) °C. Asterisks (*)

represent the non-thermolysed precursor 4. b) IR spectrum of 1a calculated at the B3LYP-

D3/6-311++G(d,p) level of theory.

The displacement vectors corresponding to the normal modes of vibration for

the prominent peaks of the radical 1a are shown in Figure 2.6.


25

Figure 2.6. Displacement vectors of the normal modes of vibration of the intense peaks of

the benzhydryl radical 1a.

The UV-vis spectrum of 1a in solid argon shows a sharp band with a maximum

absorption at 324 nm and a progression at 304 and 296 nm (Figure 2.7) and is also in

good agreement with the literature.81-83

Figure 2.7. UV-vis spectrum of the benzhydryl radical 1a generated by FVT of 4 at

(550-560) °C in argon matrix at 3 K.

ii) In amorphous water-ice: The products of the FVT of 4 together with a large

excess of water vapor were deposited on to the cold spectroscopic window at 3 K

(Scheme 2.4). IR, UV-vis and EPR measurements were performed.


26

Scheme 2.4. Generation of the benzhydryl radical 1a by the FVT of 4 at (550-560) °C in

amorphous water-ice matrix at 3 K.

The water vapor was deposited on to the cold cesium iodide CsI window to form

the amorphous water-ice matrix. Comparing the spectra with the spectra in argon, it is

observed that the amorphous water-ice is not a good choice as a matrix for the complete

isolation of the radical. Hence, along with the formation of 1a, visible amounts of the

dimer 1,1,2,2-tetraphenylethane 4 (precursor) were observed. On annealing the ice

matrix, all of the radicals dimerized to reform the precursor 4 (Figure 2.8). The IR

spectrum of 1a in amorphous water-ice matrix was recorded with only the strongest

features of 678.6, 746.1, 1022.1, 1447.7 and 1477.4 cm-1 being clearly visible (Figure

2.8(a), Table 2.2). The peaks are slightly shifted due to the nature of the matrix used.

The IR spectrum of 1a was also recorded in D2O matrix.

Figure 2.8. IR spectra of the benzhydryl radical 1a in amorphous water-ice matrix. a) IR

spectrum of showing the formation of 1a by the FVT of 4 at (550-560) °C in amorphous

water-ice matrix at 3 K. The asterisks (*) denote the precursor 4 formed back due to the


27

recombination of 1a in amorphous water-ice matrix. b) Difference IR spectrum: bands

pointing downwards are disappearing in intensity upon warming from 4 K to 110 K and are

assigned to 1a, bands pointing upwards shows the appearance of the precursor 4. c) IR

spectrum of 4 in amorphous water-ice matrix at 3 K.

The UV-vis spectrum of 1a in water matrix shows a strong and broad absorption

at 329 nm (Figure 2.9).84-87 It is red shifted to the values obtained with argon.

Figure 2.9. UV-vis spectrum of the benzhydryl radical 1a generated by FVT of 4 at

(550-560) °C in amorphous water-ice at 10 K.

2.2.3. EPR studies of the benzhydryl radical

(i) In argon matrix: The products of the FVT of 4 together with a large excess

of argon were deposited onto an oxygen free copper (Cu) rod at 5 K. The oven was

heated for a few hours to get rid of the signals due to methyl radicals obtained during

the pyrolysis experiments by default. The radical 1a exhibits very intense EPR signals

in the radical region around 330-340 mT. The experimental EPR spectrum obtained is

confidently assigned to 1a through simulation (Xsophie) with hyperfine coupling

constants aα = 14.8 G, ao = 3.7 G, am = 1.1 G, ap = 4.1 G comparable to the reported

values (Figure 2.10).88-91


28

Figure 2.10. EPR spectra showing the formation of the benzhydryl radical 1a. Black line

represents the simulated spectrum obtained by providing the hyperfine coupling constants

using Xsophie software. Dotted black line represents the experimental spectrum obtained

after FVT of 4 at (550-560) °C in pure argon matrix at 5 K.

(ii) In amorphous water-ice: EPR studies of 1a in amorphous water-ice were

performed in the same way as with argon. The EPR spectrum shows a broad unresolved

signal in the 330-340 mT region. The signal was identified as that of the radical 1a in

accordance with the literature. The broadness in the signal is due to the amorphous

water-ice matrix (Figure 2.11).


29

Figure 2.11. EPR spectrum showing the formation of the benzhydryl radical 1a by FVT of

4 at (550-560) °C in amorphous water-ice at 5 K.

Table 2.2. Experimental and calculated vibrational frequencies of the benzhydryl

radical 1a.

Mode Sym Calculateda

ν/cm-1 (Iabs)d

Argonb

ν/cm-1 (Irel)e

Amorphous

water-icec

ν/cm-1 (Irel)e

Assignment

11 B 489.0 (20.0) 486.6 (15.3) 486.5 (21.0) C-H bend (wagging)

12 B 578.0 (12.0) 563.5 (19.4) 563.8 (18.0) Ring stretch

16 B 689.0 (59.0) 675.9 (100) 677.4 (66.2) C-H bend (wagging)

17 A 698.0 (6.0) 683.8 (15.1) - C-H bend (wagging)

18 B 711.0 (32.0) 688.1 (31.2) - C-H bend (out of plane)

radical center

19 A 758.0 (5.0) 745.9 (14.1) 746.8 (42.3) C-H bend (wagging)

20 B 794.0 (56.0) 777.8 (97.2) 781.4 (100.0) C-H bend (wagging)

22 B 837.0 (1.0) 822.4 (4.2) - C-H bend(twisting)

25 B 912.0 (7.0) 894.3

896.2 (14.3) 897.9 (34.1) C-H bend(twisting)

31 B 1001.0 (1.0) 942.4 (1.1) - C=C str. Ring

33 B 1044.0 (8.0) 1024.3 (12.4) 1021.6(4.5) C-H bend (in plane)

35 B 1102.0 (1.0) 1092.4 (4.1) - C-H bend (in plane)

36 A 1120.0 (7.0) 1108.8 (6.2) 1109.3 (13.7) C-H bend (in plane)

43 A 1327.0 (1.0) 1320.0 (1.6) 1320.5 (11.9) C=C str. Ring

48 A 1474.0 (7.0) 1446.6 (39.8) 1447.5 (40.2 C=C str. Ring

49 B 1496.0 (10.0) 1473.3 (32.9) 1475.2(19.7)

Asymm. C-C-C stretch

(radical center) 50 B 1507.0 (23.0) 1478.4 (20.7)

53 A 1588.0 (2.0) 1567.3 (2.3) - C=C str. Ring

54 B 1605.0 (6.0) 1591.4 (0.7) - C=C str. Ring


30

56 A 3138.0 (7.0) 3013.1 (64.5) - C-H stretch (radical

center)

62 A 3173.0 (45.0) 3067.5 (73.4) - C-H str. Ring

63 B 3186.0 (49.0) 3077.3 (91.3) - C-H str. Ring

a Calculated at the B3LYP-D3/6-311++G (d,p) level of theory. b In argon matrix at 3 K. c In

amorphous water-ice matrix at 3 K. d Absolute intensities in km/mol. e Relative intensities

based on the strongest observed absorption band.

2.2.4. Photochemistry of the benzhydryl radical

After the deposition of the FVT products of 4 in argon, the matrix was irradiated

with lights of varying wavelengths, for example, LED ranging from 650 nm to 365 nm,

XeCl excimer laser, mercury arc lamps producing UV light and Ar-discharge lamp. Out

of these light sources none of them could excite the radical 1a to induce photochemical

changes except the XeCl excimer laser and the Ar-discharge lamp. Thus, the

photoionization of 1a was investigated in solid argon using an Ar-discharge lamp (105

nm cutoff, corresponding to 11.6 eV) and a XeCl excimer laser (308 nm, corresponding

to 4.0 eV). The ionization potential of 1a in the gas phase is calculated to be 6.5 eV at

the B3LYP-D3/6-311++G(d,p) level of theory, and therefore, the energy of the light of

the Ar-discharge lamp is more than sufficient for the ionization of 1a, whereas two

photons would be required with the XeCl laser (308 nm).

During Ar-discharge-lamp photolysis of 1a in argon matrix at 10 K, the matrix

turned slightly yellow, and a very weak visible absorption with a maximum at 443 nm

was observed. This band is close to that reported for the benzhydryl cation 2a obtained

by protonation of diphenylcarbene 3 in amorphous water-ice.83 The yield of the cation

2a obtained during the photolysis is low and does not increase after prolonged

irradiation. Slightly higher yields of 2a are obtained if the matrix-isolated radical 1a

was irradiated using a XeCl excimer laser (308 nm, 0.3 J/pulse), although the ionization

of 1a is still not efficient (Figure 2.12). These experiments demonstrate that the matrix-

isolated radical 1a can be ionized via one or two photon processes, however, the yield

of cation 2a is very low, presumably because of the ejected electrons recombining with

the cation. Such low-yields of photoionization of PAH molecules trapped in neon and

argon matrices have been well documented in the literature and are attributed to

electron-ion recombination within the matrix cage soon after ionization.


31

Figure 2.12. UV-vis spectra showing the formation of the benzhydryl cation 2a upon

irradiation of the benzhydryl radical 1a with 308 nm XeCl laser in argon matrix at 10 K

showing the inefficient conversion to 2a. The spectra on the righthand side is zoomed 10

times.

2.2.5. Photoionization in amorphous water-ice

FVT of 4 and trapping of the products with water at 4 K produced transparent

matrices of amorphous water-ice doped with the radical 1a. As described in

Section.2.2.2., radical 1a in water was identified by IR, UV-vis, and EPR spectroscopy,

and a very good match between the spectra in solid argon and amorphous water-ice was

found. The major difference is that the IR and EPR spectra in water show broader

linewidths and less fine structure than in argon. This presumably results from the less

homogeneous matrix environment in water compared to argon, as noted in earlier

publications.36, 92

Surprisingly, 1a isolated in amorphous water-ice matrix, shows excellent and

prominent photochemistry. Irradiation of 1a with Ar-discharge lamp, 308 nm XeCl

laser, 365 nm LED and 280-400 nm (with 320 nm filter) mercury arc lamp leads to the

disappearance of the radical peak and appearance of new peaks in high yields at 477.4,

561, 995, 1180, 1223.7, 1338.8, 1362.0, 1426.0, 1451.0, 1521.6 and 1580.1 cm-1. These

new peaks were assigned to the benzhydryl cation 2a as noted in earlier publications.83


32

However, the photoionization of 1a in amorphous water-ice matrix is highly efficient

with the 308-nm laser, resulting in high yields (approximately 60%) of the cation 2a

(Figure 2.13).

Figure 2.13. IR spectra of the benzhydryl cation 2a in amorphous water-ice matrix at 3 K.

a) Difference IR spectrum showing the formation of the cation 2a upon 308 nm irradiation

of the radical 1a. Peaks pointing downwards show the disappearance of 1a while peaks

pointing upwards shows the appearance of 2a together with 4 (*). b) IR spectrum of 2a

obtained by protonation of carbene 3 in amorphous water-ice matrix.93

The IR spectrum of the cation 2a in amorphous water-ice matrix obtained by the

photoionization of the radical 1a is in excellent agreement with the spectrum obtained

by protonation of carbene 3 in the same matrix, clearly indicating the formation of 2a

via two independent routes: radical ionization and carbene protonation.

The UV-vis spectrum of the formation of the cation 2a from the photolysis of

the radical 1a at 10 K further confirms the presence of 2a (Figure 2.14). This is in very

good agreement with the literature.85, 94-95


33

Figure 2.14. UV-vis spectra showing the decay of the radical 1a at 329 nm and the

formation of the benzhydryl cation 2a at 443 nm together with 4 at 261 nm on using

308 nm XeCl Laser in pure amorphous water-ice matrix at 10 K.

The color of the matrix turns yellow on irradiating the radical 1a (Figure 2.15).

This adds to the evidence of the formation of the cation 2a from the radical 1a.

Figure 2.15. Generation of a yellow colored matrix on irradiation of the benzhydryl radical

1a due to the formation of the benzhydryl cation 2a (a) in the UV matrix cold head (b) in

the IR matrix cold head.

If 1a is isolated in a D2O matrix, 308 nm photolysis leads to 2a without

incorporation of deuterium, as expected (Figure 2.16).

a b


34

Figure 2.16. IR spectra of the benzhydryl cation 2a in different matrices at 3 K. Difference

IR spectrum of 2a in (a) amorphous water-ice and (b) D2O matrix obtained after 308nm

XeCl laser irradiation of the benzhydryl radical 1a at 3 K. Peaks pointing downwards show

the disappearance of 1a while peaks pointing upwards shows the appearance of 2a together

with the formation of 1,1,2,2-tetraphenylethane 4 marked with asterisk (*).

In contrast, if 2a is synthesized via carbene protonation, deuterium is

incorporated in solid D2O (Scheme 2.5).

Scheme 2.5. Generation of the benzhydryl cation 2a from two different sources i.e., radical

1a and carbene 3. The radical route forms the 2a whether H2O or D2O is used. The carbene

route leads to the formation of deuterated cation on using D2O.

Prolonged exposure of the radical 1a to the 308 nm Laser/365 nm LED/320-400

nm mercury arc lamp leads to the complete disappearance of 2a (443 nm) followed by


35

the decrease of the radical 1a band at 329 nm and subsequent increase of the precursor

4 peak (Figure 2.17, Scheme 2.6).

Scheme 2.6. The generation of the benzhydryl radical 1a by the FVT of 4 and its

photochemistry in amorphous water-ice matrix at 3 K.

Figure 2.17. UV-vis spectrum showing the end fate of the benzhydryl radical 1a due to

prolonged irradiation for more than 20 hours. Bold black line: UV-vis spectrum of the

benzhydryl radical 1a in amorphous water-ice matrix at 9 K with absorption peak at 329

nm. Black dotted line: The formation of 4 on prolonged irradiation of 1a with 280-400 nm

Hg lamp for more than 20 hours.

2a is a transient species and, on annealing, it disappears without the formation

of any new products.


36

2.3. Conclusion

Thus, the benzhydryl radical was successfully generated and characterized in

argon, amorphous water-ice, and D2O matrices at a temperature of 3 K in good yields.

Amorphous water-ice and D2O ice cooled to 3 K act as good electron traps and hence,

the formation of the benzhydryl cation is possible on irradiation as the ionization energy

is effectively lowered from 6.5 eV to around 3.8 eV (LED shorter wavelength limit),

unlike argon. Supplying appropriate energy to the benzhydryl radical isolated in a pool

of amorphous water-ice allows for the easy excitation of the radical and the transfer of

the electron to the amorphous water-ice to form the benzhydryl cation. Thus, organic

radicals behave similarly to neutral PAHs in that their photoionization is very efficient

in amorphous water-ice.

Prolonged irradiation with 280-400 nm Hg lamp/308 nm Laser/365 nm LED

leads to the decay of both the benzhydryl cation and radical followed by the increase in

the intensity of 1,1,2,2-tetraphenylethane. This shows that the radical-radical coupling

(dimerization) is a favored process when appropriate energy is provided.

Thus, a new route for the formation of the benzhydryl cation in neutral medium

is described here which paves the way for further interesting chemistry.

Chapter 3 Benzhydryl Cation

37

3. Benzhydryl Cation

3.1. Introduction

Ionized molecules are energy-rich and highly reactive species, making further

chemical reactions mostly barrierless processes. Even at 5 K, ionized PAHs readily

undergo hydrogenation and oxygenation (forming hydroxy-PAHs).96 Understanding

radiation-induced chemical pathways of organic matter in water-dominated ice medium

is also important for our understanding of how pollution effects Earth’s cryosphere

(polar ice caps to snow and glaciers to cirrus cloud dominated by ice grains).20-22

Finally, organic chemistry in ice medium also helps to understand the processes of

cryosolvation and radiation biology. Thus, the photoionization of organic molecules to

generate highly reactive species is the center of many complex chemical reaction

pathways both on Earth and in a wide variety of environments in our Solar System and

beyond.23-24

The ionization of organic molecules, specifically PAHs is very efficient in

water-ice medium.97-98 Both the ionized PAH (cation) and electron are stabilized in

water-ice. However, the fate of electron is still not completely understood. Riedle et. al.

described the ultrafast UV photochemistry of benzhydryl chloride, which

predominantly results in the homolytic bond cleavage and formation of a radical pair

between 1a and chlorine (Cl) atoms.90 Radical pairs with small inter-radical distances

undergo efficient electron transfer (43% in CH3CN as solvent) to form benzhydryl

cations and chloride anions with a mean electron transfer time of 22 ps (Scheme 3.1).

Interestingly, the direct heterolysis of Ph2CHCl 5 to form the ion pair is only a very

minor reaction channel.

Scheme 3.1. Ultrafast UV photochemistry of benzhydryl chloride 5 leading to the

formation of radical pair and eventually cation pair as observed by Riedle et al.

Amorphous water-ice is a polar matrix that stabilizes cations and thus reduces

the ionization potential of neutral compounds.98 Gudipati et al. studied the

Chapter 3 Benzhydryl Cation

38

photoionization of PAHs in amorphous water-ice and estimated that this matrix

stabilizes the radical cations of these hydrocarbons by up to 2 eV.99-100

Here two key questions are addressed: (a) How are organic radicals different

from neutral molecules in regard to the ionization in water-ice? Is it possible to attach

electrons to aromatic radicals forming negatively charged anions in water-ice?

Benzhydryl radical as studied in Chapter 2 was used to answer these questions. One of

the questions addressed is: does photoionization of the benzhydryl radical results not

only in the benzhydryl cation but in addition in the benzhydryl anion by electron

attachment? A major difference between ionization of a neutral molecule such as a PAH

and a radical such as the benzhydryl radical is that the former goes from a closed-shell

to an open-shell electronic configuration, whereas the latter starts with open-shell and

leads to closed-shell electronic configuration upon ionization through removal of an

electron (cations) or by the addition of an electron (anions). Hence, comparing these

two cases would help to understand the chemical evolution of organic matter in water-

ice environment.

Here, the reversible photoionization of the benzhydryl radical 1a in amorphous

water-ice matrices was studied. The efficiency of the photoionization of the radical 1a

depends on two main factors: (i) the capability of the matrix to stabilize the benzhydryl

cation 2a, and (ii) efficient trapping of the electron ejected from 1a that hind

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